It is frequently desired, either for control or diagnostic purposes, to detect the position of a motor driven actuator, such as a movable door in an automotive air conditioning duct. In applications where the actuator is driven by a brush-type DC motor, the actuator position may be reliably and inexpensively determined by detecting and counting pulses in the motor current caused by the periodic commutation of motor current by the motor brushes. In general, the pulses are extracted by filtering, and compared to a threshold to distinguish commutation pulses from noise pulses. When a commutation pulse is detected, a one shot is triggered to produce a logic level pulse, and the one shot pulses are counted to provide an output corresponding to the actuator position.
Several different pulse detection circuits have been proposed. In one such circuit, described in U.S. Pat. No. 5,132,602, a resistive shunt is connected in a ground path of the motor drive circuit, and the voltage across the shunt is capacitively coupled to the filter circuit. In another circuit, described in U.S. Pat. No. 5,514,977, a resistive shunt is connected in series with the motor, and the voltage at a node between the motor and shunt is capacitively coupled to the filter circuit. In still another circuit, described in co-pending U.S. patent application Ser. No. 09/249,339, filed Feb. 12, 1999, a high impedance circuit connected across the motor controls the current in a sense resistor in proportion to the motor current, and the sense resistor voltage is provided as an input to the filter circuit. Alternatively, the motor voltage itself may be capacitively coupled to the filter circuit, as also described in the co-pending U.S. patent application Ser. No. 09/249,339.
A problem experienced in each of the above-described circuits concerns reliably distinguishing commutation pulses from noise pulses. The problem occurs particularly with those circuits which are designed to detect commutation pulses both during motor running and motor braking or coasting, because the pulse amplitudes are much higher during running than during braking or coasting. For example, the commutation pulse amplitude during motor running may be 50 mV or more, while the amplitude during motor braking or coasting may be as small as 14 mV. To detect all commutation pulses, the comparator circuitry is generally designed with a detection threshold slightly lower than the smallest expected pulse, say 12 mV. However, the susceptibility to noise increases as the detection threshold decreases, resulting in an increased likelihood of erroneous pulse detection. What is needed is a simple detection circuit that is insensitive to noise pulses, but will reliably detect all commutation pulses.